In the forthcoming evolution of the mobile cellular standards like Global System for Mobile Communication (GSM) and Wideband Code Division Multiple Access (WCDMA), new transmission techniques like Orthogonal Frequency Division Multiplexing (OFDM) are likely to occur. Furthermore, in order to have a smooth migration from existing cellular systems to the new high capacity high data rate system in existing radio spectrum, the new system has to be able to operate in a flexible bandwidth. A proposal for such a new flexible cellular system is Third Generation (3G) Long Term Evolution (3G LTE) that can be seen as an evolution of the 3G WCDMA standard. This system will use OFDM as multiple access technique (called OFDMA) in the downlink and will be able to operate on bandwidths ranging from 1.25 MHz to 20 MHz. Furthermore, data rates up to 100 Mb/s will be supported for the largest bandwidth.
One important aspect of 3G LTE is the mobility function, hence synchronization symbols and cell search procedures are of major importance in order for user equipment to detect and synchronize with other cells. Cell search is a procedure by which a user equipment can find a cell for potential connection to. In this procedure, the user equipment detects the identity of the cell and estimates the frame timing of the identified cell. The cell search procedure also provides estimates of parameters needed for reception of system information on the broadcast channel. In 3G LTE, 510 different cell identities are supported, which can be divided into 170 cell identity groups of three identities each.
It is noted that 3G LTE supports both frequency- and time-division-based duplex. Frequency Division Duplex (FDD) implies that downlink and uplink transmission takes place on different frequency bands, while Time Division Duplex (TDD) implies that downlink and uplink transmission take place in different, non-overlapping time slots.
The time domain structure for 3G LTE transmission defines a frame of 10 ms length consisting of ten equally sized subframes of length 1 ms. Each 1 ms subframe consists of two equally sized slots of length 0.5 ms, and each slot consists of a number (normally seven) of OFDM symbols. The first and sixth subframes of each frame include synchronization signals, which are transmitted on the downlink of each cell for use in the cell search procedure. A primary synchronization signal and a secondary synchronization signal are provided, which for FDD are specific sequences that are inserted into the last two OFDM symbols in the first slot of the first and sixth subframes. For TDD, the secondary synchronization signal is transmitted in the last symbol of the first and sixth subframes, and the primary synchronization signal is transmitted in the first symbol of the next slot.
As the transmission is based on OFDM, the basic LTE downlink physical resource can be seen as a time-frequency resource grid, where each resource element corresponds to one OFDM subcarrier during one OFDM symbol interval. The subcarriers are grouped into resource blocks, where each resource block consists of 12 consecutive subcarriers during a 0.5 ms slot, i.e. each resource block consists of 12·7=84 resource elements in the normal case.
To enable channel estimation, known reference symbols are inserted into the OFDM time-frequency grid. They are inserted within the first and the third last OFDM symbols of each slot and with a frequency-domain spacing of six sub-carriers, and there is a frequency-domain staggering of three subcarriers between the first and second reference symbols. Thus there are four reference symbols within each resource block. The reference symbols are also referred to as CQI (Channel Quality Indicator) pilots.
The complex values of the reference symbols will vary between different reference symbol positions and between different cells. The LTE reference signal sequence can be seen as an indicator of the cell identity. Each reference signal sequence can be considered as a product of a two-dimensional pseudo-random sequence and a two-dimensional orthogonal sequence. The LTE specification defines a total of 170 different pseudo-random sequences, each corresponding to one out of 170 cell identity groups. There are defined three orthogonal sequences, each corresponding to a specific cell identity within each cell identity group.
As mentioned, a cell search procedure is used by a receiver to detect and synchronize to a new or another cell. To reduce the cell search complexity, the currently proposed cell search scheme for LTE is done in several steps.
In the first step, the primary synchronization signal is used to detect the timing on a 5 ms basis for a new cell. Since the primary synchronization signal is transmitted twice in each frame, it can only provide the frame timing with a 5 ms ambiguity.
In the second step, the frame timing is determined from the secondary synchronization signal by observing pairs of slots where the secondary synchronization signal is transmitted. Also the cell identity group is detected in this step.
Next, in the third step, the full cell identity is detected from the reference symbols by determining the orthogonal sequence used for the transmitted reference symbols. This is done by correlating a received sequence of reference symbols with each one of the possible known orthogonal sequences and identifying the received as the known sequence giving the largest correlation result.
Finally, when the cell search procedure is complete, the Broadcast channel is read to receive broadcasted cell specific system information.
The first two steps are well known in the art and similar to the cell search scheme in WCDMA. Also a step similar to the third step is used in WCDMA, where the pilot signal (CPICH) is scrambled with a pseudo-random sequence determining the cell ID. By assuming that the channel that affects the pilot signal over a certain interval (one to two slots in WCMDA) is constant, the scrambling sequence can be detected easily. In 3G LTE, the idea is to scramble the reference symbols with a pseudo-random sequence to discriminate between cells in different cell groups and then apply orthogonal sequences on the reference symbols, where the orthogonality is within the cell group. However, in contrast to WCDMA, LTE does not have strong continuous pilots channel, but relies on fewer reference symbols as mentioned above. These reference symbols are placed in the first and third last OFDM symbol in each subframe, and placed on every sixth carrier, i.e. with a distance of 90 kHz between the pilots.
A problem using pilot symbols that are transmitted on different sub-carriers for scrambling code identification is that the phases for the different sub-carriers typically are affected in a different and unknown way. This means that coherent alignment of the pilots is not feasible without channel equalization making the scrambling detection procedure much harder in LTE.
Furthermore, in order to have coherence gain, reference symbols used for cell identity detection will be spread out on a relative long time scale (1 ms) making the cell identity detection also sensitive to frequency errors. Frequency errors may make it difficult or even impossible to determine the orthogonal sequence used for the transmitted reference symbols, since the correlation results may be severely affected by a frequency error. Also in other communications systems, a received sequence needs to be identified, even in case of frequency error.
Consequently, there is a need for a way of identifying a received sequence as one of a number of known sequences, which is robust against frequency errors.